1. Field of the Invention
The present invention relates to a method of analyzing a subject for excessive daytime sleepiness, and more particularly to a quick (short duration), quantitative method of analyzing a subject for sleep apnea. The present invention additionally relates a device for use in detecting excessive daytime sleepiness (EDS) and another device for detecting sleep apnea.
2. Technical Background
Nearly one in seven people in the United States suffer from some type of chronic sleep disorder, and only fifty percent (50%) of people are estimated to get the recommended seven (7) to eight (8) hours of sleep each night. It is further estimated that sleep deprivation and its associated medical and social costs (loss of productivity, industrial accidents, etc) exceed $150 billion dollars per year. Excessive sleepiness can deteriorate the quality of life and is a major cause of morbidity and mortality due to its role in industrial and transportation accidents. Sleepiness further has undesirable effects on motor vehicle driving, employment, higher earning and job promotion opportunities, education, recreation, and personal life. The National Traffic Safety Board estimates that annually there are 100,000 crashes involving fatigue, resulting in 71,000 injuries and 1,500 fatalities. The National Sleep Foundation has recently conducted a poll that revealed that 17% of drivers indicated they nodded off. The numbers were even higher in shift workers, with 20%-30% reporting a fatigue-related driving mishap in the past year.
Excessive daytime sleepiness (EDS) is a symptom describing an increased propensity to fall asleep, often during monotonous or sedentary activities. EDS and fatigue are different. A subject suffering from fatigue senses a physical weakness or lack of energy, and may not have an increased propensity to fall asleep at an inappropriate time. The underlying etiology of EDS generally falls into three categories: lifestyle/work schedule, primary sleep disorders, and secondary sleep disorders. Lifestyle and work schedule disorders result from the demands of our modern society. This lifestyle results in only an estimated 50% of people in the US getting the required 7-8 hours of sleep every night. Secondary sleep disorders include loss of sleep due to pain associated with chronic infections, neurological/psychiatric disorders, or alcohol/substance abuse disorders. Primary sleep disorders affect approximately 50 million Americans of all ages, and include narcolepsy, restless legs/periodic leg movement, insomnia, and most commonly, obstructive sleep apnea (OSA). OSA's prevalence in society is comparable with diabetes, asthma, and the lifetime risk of colon cancer. OSA is grossly under diagnosed; an estimated 80-90% of persons afflicted have not received a clinical diagnosis.
EDS is currently diagnosed via two general methods. The first is via subjective methods such as the Epworth and Stanford Sleepiness Scale, which generally involve questionnaires asking a series of qualitative questions regarding subjects' sleepiness during the day. With these methods, however, it is found that the patients usually underestimate their level of sleepiness or deliberately falsify their responses because of their concern regarding punitive action, or as an effort to obtain restricted stimulant medication.
The second is via physiological based evaluations such as all night polysomnography to evaluate the patients sleep architecture (e.g., obtaining a respiratory disturbance index to diagnose sleep apnea) followed by an all day test such as the Multiple Sleep Latency Test (MSLT) or its modified version, Maintenance of Wakefulness Test (MWT). The MSLT consists of four (4) to five (5) naps and is considered the most reliable objective measure of sleepiness to date. The MSLT involves monitoring the patient during twenty (20) to forty (40) minute nap periods in two-hour intervals to examine the sleep latency and the sleep stage that the patient achieves during these naps, i.e., the time it takes for the patient to fall asleep and the deepness of the sleep. A sleep disorder such as narcolepsy, for example, is diagnosed when the patient has a restful night sleep the night before but undergoes rapid eye movement sleep (REM sleep) within five (5) minutes of the MSLT naps. The MWT is a variation of the MSLT. The MWT provides an objective measure of the ability of an individual to stay awake. The MWT is an all-day test of wakefulness that is performed under identical conditions to MSLT, but where the latency of sleep is determined after the patients are instructed to attempt to remain awake.
While the MSLT and MWT are more objective and therefore don't have the same limitations as mentioned for the subjective tests, the MSLT and MWT have their own limitations. Both the MSLT and MWT require an all-day stay at a specialized sleep clinic and involve monitoring a number of nap opportunities at two hour intervals throughout the day. In addition, both tests are more demanding than the overnight polysomnogram because the technician must monitor the naps and determine the sleep onset in real-time. Further, MSLT is usually not performed in OSA patients because the test is mostly prescribed for non-respiratory sleep disorders, since the mean sleep latency results from MSLT are not linearly correlated with sleepiness. Finally, studies have shown that the MSLT is not particularly suited for gauging the effects of therapeutic intervention. This was demonstrated in studies by Thorpy in 1992 and Van den Hoed et al. in 1981 showing no reliable reduction in sleepiness in patients given stimulant medications for narcolepsy.
The MWT was developed in 1982, in part, to address some of the short-comings of the MSLT method. The MWT, however, created another problem, the “ceiling effect.” This is the tendency of less “sleepy” individuals to perform the MWT without falling asleep. In fact, the length of the MWT trial was lengthened from twenty (20) to forty (40) minutes in 1984 because it was observed that patients with histories of excessive daytime sleepiness were too often able to maintain wakefulness for the twenty (20) minutes. In addition, while the MSLT and MWT are objective and “broadly” quantitative tests in that they both require the patient to fall asleep during the test and they measure the number of sleep incidents during the testing regiment, these tests are too costly and lack the degree of quantitative resolution necessary to easily permit measurement of effects of therapeutic intervention and degrees.
Despite the enormity of the problem, the current methods of assessing sleepiness are limited in their utility and accuracy, as indicated in the recent research plan and recommendations of the US National Center on Sleep Disorders Research. Currently, there are a number of subjective assessments (self-reports) including Stanford Sleepiness Scale, Epworth Sleepiness Scale, etc. However, the accuracy and reliability of these questioners are limited because people usually underestimate their level of sleepiness or they deliberately falsify their responses (e.g., to avoid punitive actions). On the other hand, the only physiologically-based (EEG and other biosignals) objective and clinically validated test of sleepiness, multiple sleep latency test (MSLT), is very expensive and time consuming (requires an all-day stay in a specialized sleep clinic) and is mostly utilized for diagnosis of narcolepsy. Other objective tests are mainly behavioral (i.e., no EEG measurement) and measure sleepiness based on accuracy and speed of the subject's response to a series of visual cues (e.g., psychomotor vigilance test, PVT).
In addition, OSA is a serious problem that affects up to 4% of the male population and perhaps up to 90% of OSA patients have not yet received a clinical diagnosis. What is needed is a test to screen for excessive daytime sleepiness and more particularly for sleep apnea. This test, unlike MSLT/MWT, should be quick, cheap, broad in its utilization, and convenient (MSLT/MWT is costly, does not routinely evaluate patients with sleep-disordered breathing, and is limited to specialized sleep labs). It is therefore an object of the present invention to develop a test which screens subjects for EDS. It is further an object of the present invention to screen subjects for sleep apnea. It is still further an object of the present invention to screen subjects for OSA. It is still further an object of the present invention to develop a test, which is of short duration. It is even still another object of the present invention to develop a test that is inexpensive. It is even still another object of the present invention to develop a test which is easily portable. It is even still another object of the present invention to evaluate patients with sleep-disordered breathing.